The present invention is directed to a narrow band or narrow line width laser transmitter which comprises a laser, particularly a semiconductor laser, and an external optical resonator coupled to the laser for generating very narrow line widths.
Monomode laser transmitters, which have a very narrow line width, are required in optical transmission systems comprising heterodyne or heterodyne receivers. In laboratory experiments, this is currently achieved with standard semiconductor lasers which are improved in terms of their spectral properties by coupling to an external optical resonator, grating or Fabry-Perot filter. A typical structure of this type comprises an external grating for rough adjustment of the laser wavelength and a Fabry-Perot etalon for fine adjustment as disclosed in an article by R. Wyatt and W. J. Devlin, "10 kHz Linewidth 1.5 .mu.m InGaAsP External Cavity Laser With 55 nm Tuning Range", Electronics Letters, Vol. 19, No. 3, Feb. 3, 1983, pp. 110-112. Since the outlay or cost of this structure is too high and above all else the thermal and mechanical stability of this structure is insufficient, this structure with free beam propagation is unsuitable for practical systems.
A more compact structure having an external fiber resonator is disclosed in an article by F. Favre and D. Le Guen, "Emission Frequency Stability in Single-Mode-Fibre Optical Feedback Controlled Semiconductor Lasers", Electronics Letters, Vol. 19, No. 17, Aug. 18, 1983, pp. 663-665. Given this structure, however, the closely adjacent resonances of the external resonator are not selected by a narrow band filter so that the laser transmitter has a tendency to skip between neighboring resonant frequencies.
A common problem with structures having free beam propagation is that they are bulky, expensive and unstable. Therefore, the structures having a free beam propagation do not come into consideration for practical uses.